1. Field of the Invention
This invention relates to laser amplifiers and more particularly to a method and apparatus for utilizing a phase conjugator for producing a high optical quality and high power laser beam.
2. Description of the Prior Art
In the past, phase conjugation has been used to correct phase aberrations in double-pass amplifiers, beam trains and atmospheric paths. For example, U.S. Pat. No. 4,005,935 issued to V. Wang entitled, "Method and Apparatus and Providing a Phase Compensated Optical Beam" discloses an energy pulse transmitted toward a target. A portion of the energy reflected from the target is received, processed so as to be back-scattered from a Brillouin mirror, amplified and then retransmitted along a path coincident with the received beam. The back-scattered energy from the Brillouin mirror is the complex phase conjugate of the received pulse and consequently the second transmitted pulse is phase encoded such that the effects of phase perturbations encountered along the received path are substantially cancelled, and near diffraction-limited convergence of the beam upon the target is obtained.
Experimental verification of a phase conjugate Brillouin mirror in a double-pass KrF laser amplifier system has been accomplished as disclosed in the article entitled, "KrF Laser with a Phase Conjugate Brillouin Mirror" by M. C. Gower and R. G. Caro in Optics Letters, Vol. 7, No. 4 April 1982. The phase-conjugate Brillouin mirror was used to compensate for aberrations and to produce a diffraction-limited output beam.
Phase conjugation via stimulated Brillouin scattering in CH.sub.4 gas has been used to correct amplifier aberrations in a Nd:YAG oscillator/amplifier system. (See "Performance of a Nd:YAG Oscillator/Amplifier with Phase Conjugation via Stimulated Brillouin Scattering" by I. D. Carr and D. C. Hanna, Applied Physics B Vol. 36, pages 83-92 (1985)).
The use of a frequency shifter with a double-pass/amplifier phase conjugation system to correct for optical distortions caused by the optics and laser amplifier to produce a nearly diffraction-limited high-power light beam impinging upon a target has been disclosed. U.S. Pat. No. 4,321,550 issued to V. Evtuhov entitled, "Phase Conjugate Correction for High Gain Amplifier Systems". (See also the article entitled, "Can Phase Conjugate Resonators Enhance Laser Performance?" by C. R. Giuliano, R. C. Lind, T. R. O'Meara and G. C. Valley, Laser Focus, (February 1983) pages 55-64.) Frequency shifting is important only in the event the stimulated Brillouin scattering reflection shifts the frequency outside the linewidth of the amplifier.
All of the above-mentioned double-pass/amplifier systems utilize either a partially reflective or polarizing beam splitter to separate the input and output beams. In a high-power system the beam splitter could likely be damaged by the high-power output beam.
The article entitled, "Applications of Optical Phase Conjugation" by C. R. Giuliano, Physics Today (April 1981) FIG. 7, page 34, discloses the utilization of polarization beam splitters to separate reference and phase conjugate waves. A reference wave of one polarization is transmitted through a first and then a second polarization beam splitter. It is then phase conjugated and its polarization altered such that the return beam is reflected by the second polarization beam splitter onto a separate path containing a focus control and tilt control. The beam is then reflected by the first polarization beam splitter and back in the line of the reference wave. This physical separation of the reference and phase conjugated beams allows possible refocusing and beam steering.
Use of a focus control, such as illustrated in the "Physics Today" article would not be applicable with a double-pass amplifier using a phase conjugator because it would prevent the phase conjugate beam from retracing the path of the reference beam thereby preventing cancellation of the amplifier aberrations. Similarly, a large tilt would degrade the system performance.